The present invention relates in general to the support structure for so-called membraned-tube walls in boilers and, in particular, to a new and useful buckstay system for supporting the membraned-tube walls in such a way that tube failures are reduced, particularly those failures which occur because of boiler start up and cool down operations.
Boiler buckstay systems are constructed of rolled steel members and/or trusses that stiffen the boiler tube walls. The boiler tube walls are subjected to combustion gas pressures which can be either positive or negative with respect to the local atmospheric pressure. The combustion gas pressure is contained by connecting the buckstays on opposite walls by bars, rods or channels to balance the resulting tension loads (pressure firing) or compression loads (balanced-draft firing). Thermal expansion of the boiler walls is usually accommodated by various designs of links, slotted members, bolts and pins making the connection between the bars, rods or channels and the buckstays. For a general discussion of this area, the reader is referred to chapters 12 and 16 of STEAM: Its Generation and Use, Copyright 1975 by The Babcock & Wilcox Company.
A brief discussion of the structures to which the present invention is applicable can be had by referring to FIGS. 1 and 2 of the present disclosure. FIGS. 1 and 2 are perspective views of a conventional boiler corner construction in the "cold" position--i.e., the boiler pressure parts and structural members are at ambient temperature. A first wall section 10 meets a second wall section 12 at an angle to form a corner 14. Each wall section 10, 12 is comprised of multiple vertically extending tubes 16 which are spaced from and welded to each other by membrane plates 18. Fluid conveyed through the tubes 16 during boiler operation absorbs heat from the combustion gases. A buckstay system is provided on the outside of the walls 10, 12, and comprises at least one buckstay 20, 22 for each wall section 10, 12, respectively. In an actual boiler construction, buckstays 20, 22 are repeated at intervals along the vertical height of the wall sections 10, 12. The buckstays 20, 22 resist bending forces which the wall sections 10, 12 experience during boiler steady state and transient operating conditions. These bending forces are due to both external loads, such as wind and earthquake, and to boiler gas side pressure, which can be either positive or negative with respect to local atmospheric pressure.
Standoff means in the form of support lugs 24 and standoffs 26 are engaged along an inner flange 28 of each buckstay 20, 22. Relative sliding movement between the standoff means 24, 26 and the buckstays 20, 22 is permitted to accommodate thermal expansion.
In FIG. 1, two continuous tie bars or channels 30 are welded to the edge of each standoff 26. Engagement means in the form of L-shaped engagement lugs 32 are welded to the outside surface of some of the horizontally spaced tubes 16 forming each wall section 10, 12. The engagement lugs 32 are welded in facing pairs to form a slot which closely receives each continuous tie bar or channel 30. The engagement means can also comprise a pair of clips as shown in the sub-illustration, one located above and the other below each continuous tie bar or channel 30, together with a tie bar pin. The clip is welded to two adjacent tubes 16 to form a loop that extends out beyond the outer surface of the continuous tie bar or channel 30. When the tie bar pin is inserted between the loop and the continuous tie bar 30, the latter is held in place against the wall sections 10, 12. The engagement means thus supports the weight of the buckstays 20, 22 which, in effect, hang on the wall sections 10, 12.
An end connection corner tie 34 spans the corner 14 and is welded to the continuous tie bars or channels 30. An end connection buckstay bracket 36 is welded to each end of the buckstays 20, 22 near the corner 14. A pair of end connection links 38 is pivotally connected by pins 40 between the end connection corner tie 34 and each end connection buckstay bracket 36. Suitable circular holes are provided in each member 34, 36, 38 for this purpose to allow for thermal expansion of the wall sections 10, 12. To explain, FIGS. 1 and 2 show the corner construction in a "cold" position before the tube wall sections 10, 12 have expanded. In this condition, each link 38 forms a small acute angle with the edge of its buckstay 20, 22 (the edge extending perpendicular to the plane of the wall sections 10, 12). In a "hot" condition, each of the links 38 would extend approximately parallel to the edge of its buckstays 20, 22, and the forces from one wall section 10, for example, would be transmitted to the buckstay 22 of the adjoining wall section 12.
Boiler walls constructed of welded membraned-tube panels (tubes are welded together in various geometric patterns) can be utilized to balance the combustion gas pressure loads between opposite walls in lieu of bars, rods and/or channels. Referring to FIG. 2, one such design utilizes a paddle tie 42 (a short bar welded to an adjacent boiler wall instead of a continuous bar), to connect the buckstays to adjacent membraned-tube walls that carry the buckstay system tension or compression loads. FIG. 2 thus differs from FIG. 1 in that the two continuous tie bars or channels 30 are replaced by a support bar or channel 30' separated from a corner paddle tie 42 welded at the corner 14 to the tubes 16 forming the wall section 10. A continuous tie bar or channel 30 is still provided on the wall section 12.
Buckstay systems with continuous tie bars, rods or channels on membraned-tube walls experience temperature differentials between the tie bars, etc., and tube walls that are of sufficient magnitude to cause failure in the tube walls and/or buckstay system during transient operation of the boiler (start up and cool down).
Buckstay systems with paddle ties have relatively few temperature differential problems. However, it is difficult and, sometimes, impossible to distribute large, concentrated combustion gas pressure loads from the rolled members, etc. through the short bars into the adjacent membraned-tube wall.
Some buckstay and membraned-tube wall attachment structures are disclosed in U.S. Pat. Nos. 4,721,069; 4,499,860; 4,395,860; and 4,059,075. While these references disclose mechanisms for accommodating expansion and contraction of the membraned-tube wall, they do not teach an arrangement for avoiding failures in the wall near a corner of the wall construction.
Corner support arrangements for a membraned-tube wall are disclosed in U.S. Pat. Nos. 4,008,691 and 3,479,994 in conjunction with solid structures that extend across the corner.